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Automotive Battery Diagnosis with a Panel Voltmeter and Current Meter (ammeter)

April 21, 2014

I have a 10 year old Saab that had some....starting issues today.  The battery was dead, but I did not know the root cause.  Before rushing out to buy a battery, there are a couple things that can cause this.

1) Bad alternator.  Sometimes called a generator, the alternator powers the car's electronics and also charges the battery.  When they go bad, your battery will not charge.  

2) Key-off battery drain.  When you turn off a modern car, there is always some current drain for the electronics.  Sometimes things go wrong, and modules do not go to sleep.  Newer vehicle's electronic systems are generally a series of modules, and sometimes one will not shut down.  This will drain the battery and cause your car to not start.

3) Damaged battery.  Eventually, all batteries fail.  The capacity of the battery drops as it gets older.  Sometimes an individual cell will die, so in the case of a 12 volt battery with 6 cells, you might notice that it is only putting out 10 volts.  Think of a battery as a cup of water that gets filled when it gets charged.  With a bad battery, your cup just doesn't have as much water in it.

As luck would have it, my multimeter died, so I had to improvise with a Panel Voltmeter and a Panel Current Meters, both from Adafruit.  

First things first, I verified the alternator output.  Very good, the alternator is putting our 14.2 volts:

Next, I wanted to verify there were no parasitic current draws when the car was off.  I hooked up the Panel Current Meter inline to the negative side of the battery.  Warning, don't try to start the car in this condition, as the little cable on the panel meter couldn't handle all the electrons that a car starter takes!

The red display shows that the vehicle electronics were drawing 0.15 amps right after the car was turned off.  That is a lot, and you really want the key-off draw to be below 0.02 amps.  Fortunately, the current flow dropped below 0.01 amps after a couple minutes when all the modules went to sleep.

Next, I cleaned up the terminals for good measure using some baking soda to neutralize any acid that may have leaked out.

I then hooked it back up and starting the car. start, and the battery voltage dropped to 10 volts.   It seems that our battery is indeed the culprit!  Next step, put in a new battery.

It is important to do this level of diagnosis, otherwise you will just be throwing parts at your car!

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